A paper mill converts wood, paper and/or byproducts thereof into a fibrous slurry. The slurry is treated and dried to form a continuous web. The web passes through a complex array of rolls and related apparatus at speeds in the range of 2,000-4,000 feet per minute to produce a continuous sheet of paper with appropriate dimensions, composition and surface characteristics. The paper typically is wound onto a large roll for shipment to other locations where the paper will be cut, folded, printed or the like for the intended end use of the paper.
Paper often will be coated on one or both sides to achieve a specified finish. The coating typically is applied in a prior art coating installation which may be part of a paper mill. The prior art coating installation may include an application roll and a backing roll disposed to define a nip therebetween. The prior art application roll may be partly submerged in a reservoir of a liquid coating material. Continuous rotation of the application roll transports some of the liquid coating material from the reservoir and onto a surface of the paper web passing through the nip. The prior art coating installation may further include a metering blade downstream from the nip and in contact with the surface of the paper web to which the coating has been applied. The metering blade removes excess coating liquid applied to the paper web. The coated web of material may then pass in proximity to driers which are operative to dry the liquid coating material applied to the web. Coating installations may be employed in tandem to sequentially coat opposed surfaces of the continuous web of material. The coating liquid applied at such tandem locations may be identical or different depending upon specifications for the paper.
Forces applied to the thin web passing rapidly through the nip in the prior art coating installation normally will not damage or break the fragile web. However, periodically defects in a paper web occur. These defects may be weaker then other parts of the web and can cause damage or breakage of the web. Such damage can be attributable to forces exerted in the actual nip, forces exerted by the metering blade or forces due to the abrupt changing of direction as the fragile wet web moves toward the coating driers. The breakage of the web passing through the prior art coating installation requires the entire coating installation to be shut down while the damaged web material is removed and while the paper web is rethreaded through the rolls. Even a short stoppage of the prior art coating installation is an inefficiency that can impose a very substantial cost penalty on the paper mill. It is estimated that as many as 150 breakages per month may occur in a prior art coater, and each breakage may take an hour to clear.
Another drawback of the prior art coating installation in a paper mill concerns the cost of equipment. In particular, the backing roll of the prior art coating installation must be manufacturing with great precision to ensure that a uniform surface is brought into contact with the re-wetted web of paper. The uniform surface typically is provided by a cover laminated onto the backing roll. The cover must exhibit a specified smoothness and resiliency and must withstand continuous exposure to heat and pressure without delamination. A properly covered backing roll is likely to cost about $400,000. These covered rolls often will be damaged by the heat, moisture, bleed through and pressure inevitably exerted thereon during the normal operation of the prior art coating installation. A damaged or worn backing roll must be replaced immediately to prevent excessive damage to the paper web being produced and to avoid long term shut down of the prior art coating installation in the paper mill. Consequently, most paper mills will include at least one spare backing roll. Thus, a typical prior art coating installation will have one $400,000 backing roll in use and at least one additional $400,000 backing roll sitting idle for use when needed. This clearly requires a substantial allocation of capital resources for even a large paper mill.
In view of the above, it is an object of the subject invention to provide a process for coating a paper web that lessens damage or breakage to the web of paper being coated, relative to that encountered on the prior art coating systems.
It is another object of the subject invention to provide a less expensive coating installation.
It is a further object of the subject invention to provide a coating installation that is less likely to damage the web of paper material being coated.